Fix NOT NULL constraint violations in merchant_staff table:
- Add permissions = 0 (default no permissions)
- Add user_id = Guid.Empty (placeholder until linked)
- Verified: CREATE→UPDATE→DELETE all working
- Add PUT /api/bff/products/{id} endpoint with ownership validation
- Add UpdateProductAsync to PosDataService
- Add edit button (pencil icon) on each product card
- Form supports edit mode: title/button text changes, pre-fills values
- Add EditProduct and SaveProduct methods in ShopPage.razor
- Full CRUD: Create, Read, Update, Delete all functional